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Types of Phase-Shifting Mask (PSM)
1. Binary Masks
2. Attenuated Phase-Shift Mask (AttPSM)
3. Alternating Phase-Shift Mask (AltPSM)
Binary Masks
A Binary photomask is composed of quartz and chrome
features. Light passes through the clear quartz areas and is blocked by the
opaque chrome areas. Where the light hits the wafer, the photoresist is
exposed, and those areas are later removed in the develop process, leaving
the unexposed areas as features on the wafer.
As feature sizes and pitches shrink, the resolution of the projection
optics begins to limit the quality of the resist image. In the example
above, there is significant energy (and intensity, which is proportional to
the square of the energy) even below the opaque chrome areas, due to the
very close proximity of the neighboring clear quartz areas. This "unwanted"
energy effects the quality of the resist profiles, which are ideally
vertical. Therefore phase-shift techniques are designed to "sharpen" the
intensity profile, and thus the resist profile, which allows smaller
features to be printed.
Attenuated Phase-Shift Mask (AttPSM)
Phase-Shift Mask (PSM) technology has been pioneered
in recent years to extend the limits of optical lithography. PSM technology
is divided into 2 categories: Attenuated (with the extreme example being
CPL™) and Alternating.
Attenuated Phase Shift Masks (AttPSM) form their patterns through
adjacent areas of quartz and, for example, molybdenum silicide (MoSi).
Unlike chrome, MoSi allows a small percentage of the light to pass through
(typically 6% or 18%). However, the thickness of the MoSi is chosen so that
the light that does pass through is 180° out of phase with the light that
passes through the neighboring clear quartz areas. The light that passes
through the MoSi areas is too weak to expose the resist, however the phase
delta serves to "push" the intensity down to be "darker" than similar
features in chrome. The result is a sharper intensity profile which allows
smaller features to be printed on the wafer.
The faint aerial image formed by the attenuated features is 180° out of
phase and thus "darker' than similar chrome features (as in binary masks).
Alternating Phase-Shift
Mask (AltPSM)
Phase-Shift Mask (PSM) technology has been pioneered
in recent years to extend the limits of optical lithography. PSM technology
is divided into 2 categories: Attenuated (with the extreme example being
CPL™) and Alternating.
AltPSMs employ alternating areas of chrome and 180 degree-shifted quartz
to form features on the wafer. AltPSM is a powerful but complex technology.
The process of manufacturing the mask is considerably more demanding and
expensive than that for Binary masks. Furthermore, the AltPSM must be
accompanied by a second "Trim" mask, resulting in extra cost and decreased
stepper throughput.
Chrome lines on the reticle are bordered on one side by quartz of phase
0°, and on the other side by quartz of phase 180°. As the phase goes from
positive to negative, it passes through 0. The intensity (proportional to
the square of the phase) also goes through 0, making a very dark and sharp
line on the wafer.
from ASML web site
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